The present invention relates to a piston for an internal combustion engine, and, more particularly, relates to a piston for an internal combustion engine which incorporates a lubricant supply cooling system for cooling the piston during operation of the engine.
A piston for an internal combustion engine generally becomes very hot during use, and is subjected to relatively severe thermal stresses as compared to other engine parts, especially on its top wall or crown portion which is directly exposed to the heat of the gases in the combustion chamber partly defined by the piston. This problem of heating of the crown of the piston has become more and more severe with modern internal combustion engines, due to increases in thermal load on the parts thereof arising from increases in engine power output. Various schemes have been developed in the past for aiding with the cooling of such a piston; and nowadays some form of active cooling for the piston is coming to be quite necessary. In particular, the concept of cooling the piston crown from below by injecting a flow of engine lubricant from the crank chamber side up into the cup shaped space defined by the piston crown and the piston skirt, so as to impinge against the lower side of the piston crown and to cool it, has been put forward in the past in various forms.
In its simplest form, such a piston cooling system does not require any special modification to the piston structure, but, in order to further improve the effectiveness of cooling of the piston crown portion, there have been published various other proposals, in which a cavity is formed in the piston crown portion, or a lubricant receiving member is mounted in the cup shaped space defined by the piston skirt and crown portion. In these constructions, the lubricant injected towards the piston crown is temporarily received into some reservoir defined in the neighborhood of the lower side of said piston crown, so as to be splashed up thereagainst as the piston passes over its top dead center and starts to be accelerated downwards in the cylinder bore. For example, such piston cooling constructions have been proposed in Japanese Utility Model Applications Nos. 49-96796 and 55-42967, which have been respectively published as Japanese Utility Model Laying Open Publications Nos. 54-26424 and 57-156052, in Japanese Patent Application No. 58-138183, and in Japanese Utility Model Applications Nos. 58-164040 and 58-188456.
However, such prior art type of constructions have not been entirely adequate for requirements, because, firstly, it is undesirable to form the piston as an assembly with a lubricant receiving member mounted therein, due to considerations of durability and vibration, and, secondly, the provision of a large cavity in the piston crown portion has tended to weaken the piston, and to subject its outer peripheral portions to higher thermal stressing, due to the restricted path available for thermal conduction from the upper surface of the piston crown portion to the skirt portion and the piston pin boss portions of the piston. In particular, the piston rings likewise tend to be subjected to undue thermal stress, and their durability is undesirably deteriorated. Yet, it is not practicable to provide any moving parts to the piston, and the construction is absolutely required to be simple and strong and reliable, in view of the vibration and accelerative forces to which it is subject during operation of the engine.